1. Field of the Invention
This invention relates to a stable secondary battery with a higher energy density and to an active material used therein.
2. Description of the Prior Art
As markets for a note-type personal computer and a mobile telephone have been rapidly expanded, there have been increased needs to a small and large-capacity secondary battery with a higher energy density used in these devices. To satisfy the needs, a secondary battery has been developed, which utilizes an electrochemical reaction associated with charge transfer on alkali-metal ions as a charge carrier such as lithium ions. Among others, a lithium-ion secondary battery has been used in a variety of electronic devices as a stable and large-capacity secondary battery with a higher energy density.
Such a lithium-ion secondary battery uses a transition-metal oxide containing lithium in a positive electrode (cathode) and carbon in a negative electrode (anode) as active materials, and performs charge and discharge utilizing insertion in and elimination from these active materials.
However, since the lithium-ion secondary battery uses a metal oxide with a large specific gravity particularly in a positive electrode, it has an insufficient secondary battery capacity per a unit weight. There have been, therefore, attempts for developing a large-capacity secondary battery using a lighter electrode material. For example, U.S. Pat. Nos. 4,833,048 and 2,715,778 have disclosed a secondary battery using an organic compound having a disulfide bond in a positive electrode, which utilizes, as a principle of a secondary battery, an electrochemical oxidation-reduction reaction associated with formation and dissociation of a disulfide bond. The secondary battery uses electrode materials comprising elements having a smaller specific gravity such as sulfur and carbon as main components. Although these materials are effective to some degree in providing a large-capacity secondary battery with a higher energy density, it has a small efficiency in reformation of a dissociated bond and exhibits insufficient stability in a charge or discharge condition.
Furthermore, there has been suggested a secondary battery also utilizing an organic compound, i.e., a secondary battery using a conductive polymer as an electrode material. It is a secondary battery whose principle is doping and undoping reactions of electrolyte ions on the conductive polymer. The doping reaction as used herein is a reaction of stabilizing excitons such as charged solitons and polarons generated by oxidation or reduction of a conductive polymer by counter ions. On the other hand, a undoping reaction as used herein refers to a reaction which is opposite to the above reaction and in which excitons stabilized by counter ions are electrochemically oxidized or reduced. U.S. Pat. No. 4,442,187 has disclosed a secondary battery using such a conductive polymer as a positive electrode or negative electrode material. The secondary battery is constituted with elements with a lower specific gravity such as carbon and nitrogen, and thus has been expected to be developed as a large-capacity secondary battery. A conductive polymer, however, has a property that excitons generated by oxidation or reduction are delocalized over a wide region of -electron conjugated system and interacted with each other. It results in a limitation to a concentration of excitons generated, and therefore, to a capacity of a secondary battery. Thus, a secondary battery using a conductive polymer as an electrode material is effective to some degree in terms of weight reduction, but is not adequately effective in terms of increase in a capacity.
As described above, there have been various proposals for a secondary battery which does not use a transition-metal containing active material, in an attempt to achieve a large-capacity secondary battery. There have been, however, provided no stable secondary batteries with a higher energy density and a large capacity.
As described above, in a lithium-ion secondary battery using a transition metal oxide as a positive electrode, a specific gravity of the element is so high that it has been theoretically difficult to prepare a secondary battery with a larger capacity than that currently used. An objective of this invention is, therefore, to provide a novel stable secondary battery with a higher energy density and a larger capacity.